The present invention generally relates to self-attaching fasteners and, more particularly, to clinch nuts.
Self-attaching fasteners are used in many industries such as, for example, the automotive and appliance industries to secure various components to metal members. A clinch nut typically includes a central pilot or punch portion, which at least partially extends into an opening in a metal plate or panel member. If the clinch nut is self-piercing, the central pilot portion cooperates with tooling to form the opening in the metal panel as the clinch nut is attached to the metal member.
The clinch nut is attached to the metal member by a die member, which forms a mechanical interlock between the clinch nut and the metal member. The die member typically deforms the metal member about the opening into contact with the pilot portion and adjacent bottom end face of the clinch nut. The pilot portion of the clinch nut may be provided with an undercut and the bottom end face may be provided with an annular groove and/or lugs to enhance the engagement with the metal member. For example, SPAC® brand clinch nuts are provided by RB&W Manufacturing LLC containing such features. The clinch nuts and the mounting thereof to metal members are illustrated in U.S. Pat. Nos. 6,220,804 and 6,409,444, the teachings of these patents being incorporated herein by reference.
When clinch nuts are attached to the metal members, screws or bolts are threaded into the clinch nuts and tightened to prescribed torque values. To that end, the clinch nut is provided with a central bore that is threaded for engagement with such screws or bolts. The central bore can also be sized to be threaded by engagement with a thread forming screw during the final assembly or installation so that different threads may be formed in accordance with specific fastening requirements. It is also desirable to form the screw thread after the clinch nut has been affixed to the metal member to avoid distortion of the screw thread during the nut mounting process.
In order to provide lighter weight products of comparable strength, there is a tendency to form the panel or plate members of high-strength low alloy (HSLA) steel, ultra high-strength low alloy (UHSLA) steel or martensitic HSLA steel. Such high-strength materials have a significantly increased hardness ranging in value from Rb 80 up to Rc 45 and require clinch nuts of increased compressive strength to assure self-piercing and/or attachment without undesirable deformation or distortion of the fastener as it is affixed to the metal member. That is, increased piercing loads and setting forces to deform the metal member into engagement with the clinch nut are required to mount the nut to the higher strength and harder metal members. Such increased piercing and setting forces may result in unwanted deformation or distortion of the fastener.
The hardness and strength of the clinch nut may not be simply increased to resolve the foregoing problems without loss of the manufacturing flexibility of thread forming by engagement with a self-threading screw. This is true since self-threading screws are intended for use with relatively ductile metals having hardness values in the range of less than 32 Rc.
It is not desirable or competitive to provide specialty self-threading screws useful with metals of differing higher hardness values and/or clinch nuts with a variety of hardness values for use with metal members of differing strength and hardness properties. This has heretofore resulted in a trade-off between the required increases in compression strength of clinch nut, especially self-piercing nuts, and the manufacturing flexibility of subsequent thread formation by engagement of the mounted clinch nut with a thread forming screw. That is, a limited number of clinch nuts of differing hardness values may be used with a number of different metal members.